1. Field of the Invention
The present invention relates to a playing apparatus which uses an optical recording medium, particularly an optical waveguide recording medium having an optical waveguide provided with refractive index-discontinuous portions to guide a laser beam (generally termed, including low coherent light beam) thereon to produce reflection of the guided beam having a plurality of different amplitudes and phase delays to perform optical heterodyne detection of the reflected part of the guided beam and Doppler-shifted laser beam, in order to reproduce recorded information as a time-series signal waveform.
2. Description of the Related Art
As a conventional optical recording medium there is an optical disk which has a plurality of low-reflectance recesses aligned in series as recorded information on a high-reflectance flat reflecting film that is formed as a recording film on a disk-shaped substrate. With this optical recording medium in use, a laser beam is irradiated and converged on the row of recesses to detect the difference between the amount of reflected light from the reflecting film and that of the recesses as recorded information. Another type of optical recording medium is a photomagnetic disk which has a plurality of minute magnetization-inverted regions aligned on a uniaxialmagnetic anisotropic recording film to record information. With this optical recording medium in use, the difference between the rotational angles of the polarizing planes of the reflected lights from the magnetization-inverted regions is detected as recorded information.
As information is reproduced from those optical recording media by utilizing the reflected light from the row of recesses or magnetization-inverted regions serving as a recording portion, there is a limit to the surface density of the recording portion. At the time of information reproduction from those optical recording media, the focal point of the laser beam is shifted along the optical axis so as to follow up the surface vibration of the optical recording media. A focusing is however necessary for each spot of the recording portion. Further, the reflectance of the reflected light and the rotational angle of the polarizing plane are very small, the signal-to-noise (S/N) ratio of an optical signal to be detected is low. In addition, as a time-series signal is reproduced only by the movement of a row of recording portions, the access time for information reproduction and recording is restricted by the moving speed of the optical recording medium.
Japanese Unexamined Patent Publication No. 2-210627 discloses an optical waveguide recording medium and a playing apparatus therefor which have been developed to overcome the above shortcomings.
A playing apparatus for an optical waveguide recording medium has been proposed which has a Michelson interferometer type optical heterodyne detecting optical system. This playing apparatus comprises a collimator lens for converting a radial laser beam from a light source into a flux of parallel light, a half mirror for causing divergence of a laser beam to be guided on an optical waveguide recording medium, an objective lens for coupling one part of the divergent laser beam to an optical waveguide, a movable mirror for causing a phase shift and a frequency shift on the other part of the divergent laser beam to provide reference light, and a photodetector for causing interference between the signal light reflected by and returning from refractive index-discontinuous portions formed on the optical waveguide and the reference light for heterodyne detection of the optical output.
Since this playing apparatus utilizes the movable mirror as a means to subject the divided laser beam to phase shift and frequency modulation for heterodyne detection, the modulation frequency is limited, thus restricting the improvement of the information density. What is more, the presence of the mirror driving section impedes miniaturization of the reproducing optical system and improvement of its reliability.